1. Field of the Invention
The present invention relates to an EL (electrolumlnescence) lamp, and more particularly, to an EL lamp having a high luminescence efficiency adapted for use in liquid crystal displays (LCD) and the like, and a process for production of the same.
2. Description of the Related Art
Recently, electronic devices have been intensively required to be lightweight, of a thin type, and workable at low voltages, thereby consuming less electric energy (workable with any electric cells), and LCDs (liquid crystal displays) have been increasingly utilized as displays. Since LCDs themselves do not generate light, back up or back lights made of EL lamps have been used in order to improve visual perceptivity of LCDs.
Such light sources have been demanded to be of a thin form, lightweight and inexpensive. EL lamps can be made as thin plane luminescence sources having a lower power consumption as compared with, for example, luminescent discharge tubes and incandescent lamps. Especially, the demand of being "thinner" is more significant in double side light emitting EL lamps.
FIG. 10 is a schematic view of an example of the prior art EL elements. On back electrode 101 made of aluminum foil is formed insulating resin layer 107. This resin layer 107 may be produced by dissolving an organic resin having a high dielectric constant (referred to as binder hereunder) in a solvent, dispersing powdery barium titanate in the solution to produce an ink-like dispersion which is applied onto back electrode 101 by a printing method or the like and dried. On this insulating resin layer 107 are formed light emitting EL layer 103 and transparent electrode 104. Light emitting EL layer 103 may be produced by dispersing a phosphor powder in a similar binder as described above to produce an ink-like dispersion which is applied and then dried. Similarly, transparent electrode 104 may be produced by dispersing an ITO (indium tin oxide) powder in a similar binder to produce an ink-like dispersion which is applied and then dried. Leads 105 are attached to and extended from back electrode 101 and the transparent electrode 104, and the bulk body is packaged with a film having a high moisture-proofing property to complete an EL element.
Barium titanate has a high dielectric constant. Even with a barium titanate layer formed between electrodes, therefore, a reduction in voltage due to the barium titanate is small. For this reason, it is possible to apply a sufficient voltage onto the light emitting EL layer, whereby a higher brightness can be easily achieved.
In order to form a barium titanate layer on the surface of an aluminum layer, however, a coating step and the like must be conducted so that a reduction in the thickness of the insulating layer is limited. Moreover, there is a difficulty that uneven coating produced in the applying step may cause a so-called "repellence" or "repulsion" resulting in uneven luminescence.
In order to overcome such difficulties as above, an attempt has been proposed to coat the surface of the aluminum foil with an alumite film, by which the aforementioned barium titanate layer can be replaced.
Japanese Patent KOKAI (Laid-open) No. Sho 64-10597 discloses a field luminescence tube with an aluminum foil back electrode which was produced by anodizing an aluminum foil, one of the surfaces of which was subjected to an alumite forming treatment to produce an insulating layer.
Japanese Patent KOKAI (Laid-open) No. Hei 1-209693 discloses an aluminum laminate for use in dispersion-type electroluminescence panels comprising an aluminum foil having an alumite layer and a white coat layer formed thereon.
Japanese Patent KOKAI (Laid-open) No. Hei 1-225097 discloses a dispersion-type EL lamp comprising an aluminum foil, the surface of which is anodized to produce a porous oxide surface film.
These techniques employ as insulating layers an alumite film which is produced on the surface of an aluminum foil for the back electrode by subjecting the foil to alumite forming treatment.
The alumite film on the surface of an aluminum foil can be produced more inexpensively than the barium titanate insulating layers, is capable of producing EL elements having an equivalent luminescence efficiency and brightness. Moreover, the aluminum foil coated with alumite film is excellent in adhesion or binding property.
FIG. 11 shows a schematic view of one of the prior art double side light emitting EL elements. The double side light emitting EL lamp was fabricated by adhering the back sides of two identical single side light emitting EL elements with each other with a common electrode being disposed between and connected to both the back sides.
Each single side light emitting EL lamp comprises back electrode 110 of an aluminum foil having insulating resin layer 117 formed thereon. Insulating resin layer 117 may be produced by dissolving an organic resin having a high dielectric constant (referred to as binder hereafter) in a solvent, dispersing powdery barium titanate in the solution to produce an ink-like dispersion which is applied onto back electrode 110 by a printing method or the like and dried.
On insulating resin layer 117 are formed light-emitting EL layer 113 and transparent electrode 114. Light-emitting EL layer 113 may be similarly produced by dispersing a fluorescent powder in a binder and mixing to produce an ink-like dispersion which is applied onto insulating layer 117 by a printing method or the like and then dried.
Similarly, transparent electrode 114 may be produced by dispersing an IT0 (indium tin oxide) powder in a binder to produce an ink-like dispersion which is applied by a printing technique onto light emitting EL lamp 113 and then dried. Alternatively, a transparent electrode film comprising a polyester film having IT0 vapor-deposited may be used as transparent electrode 114.
Common electrode lead 115 is attached to and extended from both back electrodes 110, and electrode leads 116 are attached to and extended from transparent electrodes 114, respectively.
The main element body is packaged with a film having a high moisture-proofing property (not shown) to complete an EL element.
The alumite processing which has been also employed for a long time in treatment of the surfaces of aluminum sashes and aluminum foils is one of techniques of the forming porous films having a thickness of 6 .mu.m to several hundred microns by conducting anodic oxidation in an acidic aqueous solution such as an aqueous solution of sulfuric acid.
The alumite layers produced by such techniques have a relatively low breakdown voltage or strength and exhibit a higher leakage current as the field intensity is increased. Therefore, the use of the alumite layers in EL elements may lower breakdown strengths of the elements, so that their luminescence efficiencies are not allowed to rise, because a higher electric field must be applied for increasing luminous intensity, if necessary.